The invention relates to integrated circuit components which function as a filter network, transformer or the like, and which combine inductance and capacitance in either two terminal or four terminal configurations, and more particularly in multiple-pole filter configurations.
In the past, discrete inductive and capacitive components have been interconnected to form LC resonant circuits for wave signal filtering purposes. Such circuits exhibit well defined impedance characteristics which are frequency dependent. This frequency dependence is a function of the particular inductive and capacitive elements used and is partially defined by the following mathematical relationship defining the resonant frequency of an inductive-capacitive combination: ##EQU1## f = resonant frequency, Hertz L = inductance, henrys
C = capacitance, farads
Through a proper selection of the inductance and capacitance, the particular LC combination can be made to resonate at virtually any frequency. The effect of such resonance is the ability of the inductive and capacitive components to interreact and thereby effectively cancel the imaginary components of their respective impedances so that, at the resonant frequency, an extremely low impedance exists between the elements themselves. This fact has made is possible, for example, to design high and low-pass filters which have a relatively flat impedance level through a particular band of desired frequencies and which attenuate unwanted frequencies. The degree of attenuation of unwanted frequencies is dependent upon the number of inductance-capacitance sections which are present in the filter, so that is is commonly understood, for example, that a standard Butterworth filter has an attenuation roll-off beyond a cutoff frequency which approximates -18DB per octave for each section. There are, therefore, advantages in being able to produce filters with as many sections; that is, LC combinations, as is physically possible.
Numerous difficulties, however, have impeded attempts by the prior art to achieve multiple section filter combinations suitable for high response filter characteristics. Most of these difficulties have been a direct consequence of the inherent limitations and deficiencies of discrete inductive and capacitive components which have been used almost exclusively in the development and use of filters and related networks. Thus, for example, while the filter circuit made up of discrete components may operate satisfactorily on conducted signals, the circuit elements, and particularly the inductive elements, generate radiated interference signals, particularly magnetic signals, which are difficult to shield and often interfere with the operation of susceptible electronic equipment located adjacent the filter circuit.
Other difficulties encountered in the use of discrete component filter designs are weight and volume limitations which are unavoidable in building multiple section filters, regardless of the attempts to package the various components densely.
In addition, serious cost difficulties arise in the construction of descrete element filters, since in many instances the individual elements must be separately packaged and the overall filter circuit itself must then be packaged, so that the entire cost, including the labor of interconnecting various components, creates a serious limitation on the number of filter sections which are practical to implement for a particular design configuration.
The present invention overcomes the serious limitations mentioned above by providing an integrated capacitive-inductive circuit component which is conveniently packaged as an integrated structure, and which operates effectively as a high resolution wave filter having a large number of sections, if desired. Part of this desirable result is achieved by utilizing elements within the integrated component to operate both as inductive and capacitive elements, primarily by constructing the inductive elements to have exposed conductive surface areas which may be capacitively coupled to one another, or to separate capacitive elements.
The cost of manufacture of such an integrated circuit component is substantially reduced, as evidenced by the cost of other integrated components in the electronic art, while the potential for producing multiple-pole filter sections within a significantly reduced volume and weight is enhanced.
More particularly, the present invention accomplishes the desired results by including an inductive element in the form of a thin conductive sheet which may be shaped to form an inductance coil, and which has a surface area of substantial dimensions. A dielectric means in the form of a thin sheet of electrically insulating substance, such as a ceramic material, is placed adjacent to the surface of the inductive elements and a capacitive element or element in the form of thin sheets of electrically conductive material are placed against the dielectric sheet, so that the inductive element and the capacitaive elements are capacitively coupled through the dielectric sheet.
The basic invention may be implemented by providing a spiral roll including a first thin electrically conductive sheet and a second thin electrically conductive sheet rolled concentric to one another with a thin dielectric sheet interposed between the first and second conductive sheets. Proper terminal connections are provided to permit at least one of the electrical conductive sheets to functions as an inductance coil. The inductance coil is formed with an exposed surface area for capacitive coupling to a second conductive sheet through the dielectric sheet, so that distributed capacitance is generated along the length of the inductive element. the capacitive sheets may be conveniently interrupted to produce a plurality of capacitive sections in order to permit different terminal connections, enabling the circuit component to function as a high-pass filter, low-pass filter, and the like.
Since the entire filter combination is packaged in the component as distributed elements, typically rolled as plural sections, the emanation of spurious radiations may be substantially eliminated. In addition, the circuit component configuration described briefly above permits the place of a magnetically permeable core within the center of the inductive winding to permit an increase in the inductance of the inductive winding, and to provide a means for tuning the resulting filter.